Indicator testing in the food industry encompasses the microbiological testing of foods for the presence of quality indicator organisms such as total heterotrophic bacteria, coliforms, E. coli, environmental microorganisms belonging to the genus Listeria, yeast, and molds. In spite of recent trends to improve time to result for the detection of these indicator organisms, growth-based yeast and mold tests continue to have the longest incubation time (e.g., 5-7 days) before a result is obtained. As a result, food processors frequently have to wait up to a week before valuable product can be shipped. This results in an inefficient inventory management system.
Certain yeasts and molds require a full 7 days incubation to form observable colonies on the current conventional microbiological media. Among those are yeasts and molds commonly found in low moisture/low water activity (Aw) foods. These microorganisms are commonly characterized as osmophilic yeast and xerophilic molds. Some of the osmophilic and xerophilic microorganisms require the use of sample-suspending media (e.g., aqueous diluents) that contain dissolved solutes (e.g., carbohydrates) at concentrations that protect the microorganisms from osmotic shock when preparing the sample for growth-based testing on microbiological media. The solutes commonly used are sucrose, glucose or glycerol. For example, Abdul-Raouf et al. report the use of diluents comprising 40% or 50% (w/w) glucose or 18% or 26% (w/w) glycerol in an article entitled “Comparison of combinations of diluents and media for enumerating Zygosaccharomyces rouxii in intermediate water activity foods” (Letters in Applied Microbiology, 1994, 19:28-31), which is incorporated herein by reference in its entirety.
In addition, the semi-solid microbiological growth media used to culture xerophilic microorganisms typically comprise at least 18% (w/w) glycerol, at least about 30% (w/w) glucose or at least 20% (w/w) sucrose in order to provide the proper osmotic environment to facilitate growth of these fastidious organisms. Petri dishes containing some of these media must be conditioned in diffuse daylight before use, thereby adding additional time and steps to the testing process.
There remains a need for simpler, faster methods to detect yeast and mold microorganisms in a sample.